I am relatively new here and not a builder of lasers, but I will take a crack at your question since no one has replied yet.
There can be a slight size difference but I don't think it is an issue in the Dorcy (it isn't in mine, see below)
Whether your Dorcy Jr. build can handle RCR123s (usually 3.6V or more) depends on the driver. In general there are buck and boost drivers that lower or raise the voltage out of the battery to match what the diode needs. The driver is the middleman. I don't know anything about the drivers available on this forum. Someone familiar with the Flexdrive may be able to answer your question more fully.
Primary CR123s are usually 3V batteries, whereas most RCR123s are 3.6 or 3.7V (and can charge up to 4V or more depending on battery and charger). Some rechargeable 123s are only 3V, and I think 3V rechargeables would work wherever 3V primaries work. You need a different charger or an adjustable charger for charging 3.0V rechargeables, since a 3.7V charger will fry them. If you want this kind of battery I would recommend getting AW's protected LiFePo4 batteries (from a certain flashlight forum) and DX sku 1236 as the charger, or get some Tenergy 3.0V batteries and charger.
I have a red laser in a Dorcy Jr. host that uses the original Dorcy driver, meant for LEDs not laser diodes. This works but from what I hear it is not ideal. My 3.0V RCR123 works fine in this laser and the power seems the same as with primaries. I am afraid to try a 3.7V RCR123 in this build, since I don't want to fry the diode. (anyone tried this?)
The bottom line: if you are looking to replace your primaries with rechargeables of the same power, go for it. If you want more power, you might want to do some research into your driver to avoid frying it and/or the diode.
Hope this helps.
